Transformation of Single‐Use Plastics into Lighter Hydrocarbons via an Economical Coal Fly Ash based Zeolite Catalyst

A mixture of zeolite X and A, derived from coal fly ash (CFA), was protonated and utilized as an acid catalyst to decompose single‐use plastics (SUP) waste into lighter hydrocarbons. Low‐density polyethylene (LDPE) was selected as the source of SUP, and this study aimed to investigate the catalytic effects of the protonated CFA‐derived zeolite (H‐XA) as an economical option on the LDPE pyrolysis temperature and output. CFA converted into H‐XA through the conventional alkali fusion, followed by sonication and simplified hydrothermal and protonation processes. The impact of the alkali agent ratio on the resulting product was investigated, and outcomes were evaluated. The product with optimum properties was then selected for investigation in the LDPE pyrolysis studies. Finally, the results were compared with commercial Protonated Zeolite Socony Mobil‐5 (H‐ZSM5) and thermal decomposition without catalysts. Regarding LDPE degradation temperature, H‐XA could lower that by 97 °C compared to thermal degradation, which was almost the same as commercial H‐ZSM5 with 110 °C. The pyrograms obtained through the Pyrolysis‐gas chromatography‐mass spectrometry (Py‐GCMS) analysis indicated the elimination of heavier hydrocarbons in the presence of H‐XA and the product spectra laid in the range of C3 to C9, which was entirely different from the LDPE with hydrocarbons of varying lengths. Moreover, kinetic studies on the LDPE decomposition reactions in this work have revealed that LDPE conversion occurs at a lower activation energy level in the presence of H‐XA, which is comparable to the energy level observed when H‐ZSM5 is used as the catalyst. The synthesis of H‐XA with desirable catalytic activity, such as the ability to lower the LDPE degradation temperature and make lighter hydrocarbons as the pyrolysis output, suggests the great potential of the CFA‐based acid catalyst for use in SUP recycling. This represent a game of waste versus waste within the framework of the circular economy, where the environment is the ultimate winner.


Introduction
Single-use plastics (SUP) are defined as products with a very short life cycle that lose their value only after first use.The mechanical properties of single-use plastics, in addition to their low cost, make them an ideal choice for packaging due to their ability to offer superior protection and preservation. [1,2]Packaging films and plastic bags are considered SUP and are commonly made of Polyvinyl chloride (PVC), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE). [3,4]hey are adopted as an alternative to natural resources-based materials such as paper in food packaging, especially due to their inertness and protecting ability. [5,6]In Australia, plastic packaging comprises roughly 1 million tonnes (� 20 %) of plastic in the market, with LDPE accounting for approximately 23.3 % (233,000 � 20 %) of that total.Despite this, only 18 % of the LDPE plastic packaging was collected for recycling in the 2018-2019 period, as reported by the AUSTRALIAN PACKAGING CONSUMPTION RECYCLING DATA.Plastic packaging's short lifespan and low recycling rates present significant challenges for waste management globally and in Australia. [7]Unfortunately, the COVID-19 pandemic has further exacerbated the issue by increasing the use of single-use plastics due to the consumer fears of reusable products coupled with a decline in the recycling rate. [8]This has created a new normal for human societies which may lead to enormous problems as plastics break down into small fragments, that cause proliferation of microplastics (MPs, < 5 mm) or even smaller nano plastics (NPs, < 1 μm), which have been already found in human bodies. [9]oreover, the utilization and trade of recycled polyolefins have recently encountered more significant restrictions.While HDPE remains in high demand and has extensive applications, [10,11] other polyolefins, particularly LDPE in the form of film, face more challenges. [12]he conversion of waste LDPE, commonly used in single-use packaging, into a valuable material can benefit both the environment and the LDPE manufacturing industry. [13]Pyrolysis, a form of chemical recycling, can facilitate this process by transforming LDPE into chemical feedstocks that can be used as fuel or monomers for producing virgin polymers.16][17] According to Serrano et al., random generation of free radicals across the polymer backbone occurs during polyethylene thermal degradation (pyrolysis), which results in molecule scission. [18]In contrast, using a catalyst can convert the pyrolysis mechanism into a carbocationic mechanism, whereby carbonium ions are formed through the protonation of hydrocarbons on Brönsted acid sites and carbenium by abstracting a hydride ion by a Lewis acid site present in the catalyst.Once carbocations are formed, a range of acidcatalyzed reactions can occur over the catalyst's acid sites, including oligomerization, cyclisation, aromatization, isomerization and cracking.During this stage, both random and endchain cracking reactions may occur, and the prevalence of each reaction is chiefly dependent on the physicochemical properties of the selected catalyst.21][22] Pyrolysis of LDPE without catalysts is generally unsuitable for industrial-scale applications because the majority of the pyrolysis outputs would require additional cracking and refinery processes to become usable by industries. [23]As a result, catalytic pyrolysis has received a great deal of attention in recent years and is at the center of several research endeavors aimed at establishing its economic viability. [24,25]oth homogeneous and heterogeneous catalysts have been used in polyolefin pyrolysis, but heterogeneous solid acid catalysts such as HZSM-5 are more commonly employed. [26]This is because acidic sites on the catalyst promote carbocationic cracking and hydrogen transfer leading to more efficient pyrolysis reactions. [27]Zeolite-based acid catalysts are the most common heterogeneous catalysts used in the pyrolysis of polyolefins due to their ability to promote light hydrocarbon production [28,29] and lower the degradation temperature. [30]The high cost of producing zeolites and the relatively high risk of catalyst deactivation during pyrolysis processes have created significant challenges in the economics of the process. [31,32]As the demand for plastic recycling increases and the recycling industry thrives, there is a growing need for cost-effective methods that can be scaled up and are technically and economically viable.Therefore, it is essential to find an economical alternative resource to reduce uncertainty in this area.
Coal fly ash (CFA) is one of the resources that has been under investigation for many years.CFA is a solid residue in the form of powder, mostly spherical particles and amorphous in nature, generated during the combustion of coal in power plants; however, the method of burning and type of coal determines the physical/chemical properties of CFA. [33]The presence of crystal components such as quartz (silica) and mullite (alumina) make it a suitable source material in the synthesis of zeolite. [24]According to the American Society for Testing Materials standard ASTM C618, there are two classes of CFA, including class F, if the amount of SiO 2 + Al 2 O 3 + Fe 2 O 3 is more than 70 wt% and class C, If the amount of SiO 2 + Al 2 O 3 + Fe 2 O 3 is between 50 and 70 wt%. [34]he low cost of coal and increasing demand for energy increased the usage of coal as a source of energy, resulting in the generation of a considerable amount of CFA every year.According to Gollakota et al., most of the generated coal fly ash driven from coal-fired boilers ends up in landfills.In Australia, for example, 13.1 million tons of fly ash is generated every year, of 55 % is disposed of in landfills, potentially causing soil and water pollution. [35]Therefore, CFA is classified as waste that requires immediate action for safe disposal. [36,37][45] Bearing sustainability and circular economy in mind, CFA was selected in this work as a source for synthesizing an ecodesigned catalyst to be used in the catalytic pyrolysis of LDPE.
To utilize the available resources in CFA, alkali fusion followed by hydrothermal treatment was selected with some modifications to make it more practical and less complex.In this regard, a sonication step was added before the hydrothermal treatment, and the high-pressure factor was eliminated from this step.This method was effective in extracting mullite and quartz which exist in CFA as the source of aluminium and silicon and using them to shape zeolite crystal structure. [46]here are several variables that can affect the effectiveness of this method, including fusion temperature, time and temperature of crystallization and the alkali/CFA weight ratio.However, for the purposes of this study, the main focus was on the alkali/ CFA ratio and its impact on the crystallinity and catalytic activity of the resulting catalyst.
In addition to the characterization of the synthesised coal fly ash-based zeolite (named Na-XA), a protonation step was performed, followed by thermogravimetric analysis (TGA) to assess the catalysts' activity in pure LDPE degradation.Pyrolysis Gas Chromatography/Mass Spectrometry (Pyrolysis-GC-MS) was then used to qualitatively evaluate the effect of the catalysts on the composition of degradation output.This allowed us to assess and compare the efficiency of the synthesized catalyst in terms of its effect on LDPE degradation temperature and output.

CFA characterization
As per the observations of Jha and Singh [47] formation of a specific CFA based zeolite depends on the CFA silica-alumina molar ratio used.In this work, the CFA silica-alumina weight ratio is 2.7 suggested by the manufacturer released data sheet based on elemental analysis.
Except the primary heat treatment which causes around 4 % weight reduction and sieving, no other modification was used for CFA and particle size analysis indicates a drop in the average CFA particle size after the pre-treatment step (Table 1).
According to Querol et al. (1997), CFA's reactive amorphous alumina silicate glassy phases enable the formation of zeolites. [48]Therefore, X-ray diffraction was used for CFA to affirm the presence of such phases in the sample.Figure 1(a) depicts the XRD spectrum of CFA after the pre-treatments interpreted by the Powder Diffraction File database.Quartz, Mullite and Hematite crystal peaks as the source of Silicon, Aluminium and Iron were detected on this pattern along with an undefined amount of amorphous phase.
Figure 1(b) shows the infrared spectroscopic analysis of CFA.In the fingerprint region, CFA's FTIR spectrum exhibits two main intense bands around 460 cm À 1 and 1092 cm À 1 , characterized as TO4 tetrahedra (T=Al, Si).The band appeared at 1092 cm À 1 , corresponding to asymmetric stretching vibrations, and the one at 460 cm À 1 is attributed to the internal deformation vibrations. [49,50]The band at 778-796 cm À 1 , is attributed to the presence of quartz in the CFA, which was detected around 796 cm À 1 in this study, and mullite, the source of aluminium, appeared at about 556 cm À 1 . [51,52]It is also noteworthy that no significant band exists in the functional group region.
SEM micrograph of the CFA sample (Figure 2) shows spherical particles called cenospheres, hollow spheres made from alumina and silica. [41]There are also irregular pieces of minerals and debris of microspheres that can be observed among the CFA different particles.In this regard, Bhanarkar et al highlighted the coal combustion method, particle size and the particulate control system as the influential factors in physical characteristic of CFA. [53]esults of the BET analysis and pore size distribution Table 2 indicates minor changes in the morphology of the CFA after the pre-treatment process.It also shows a relatively small surface area of 2.7 m 2 /g for CFA after the pre-treatment, suggesting that the material is unsuitable to directly use as a catalyst for LDPE degradation.This is confirmed by the TG curve (Figure S1 appendix), which depicts no significant change in the TG curve with the presence of CFA.

Effect of alkali concentration on the synthesized zeolite properties
Effect of NaOH concentration on the fusion step was studied in 4 different NaOH to CFA ratios, including 1.2, 1.8, 2.4 and 4.8.
Fused materials then subjected to some more treatments and powdery samples were achieved at the end (Figure 3).It is noteworthy that the 1 to 4.8 ratio fusion output was stiff, stuck in the crucible and was difficult to grind.
Pores distribution and specific surface area of the synthesized materials was measured for a better understanding of the morphology at different alkali ratios.The Brunauer-Emmett-Teller (BET) and Barret-and Joyner-Halenda (BJH) model applied to analyze specific surface area and pore size distribution.The average pore diameter and specific surface area were calculated after 12 hours of degassing under vacuum at 120 °C and result shown on Table 3.
Pore diameter of the synthesized material is distributed in the range of 3 to 4 nm.The specific surface area shows an optimum of 431 m 2 /g in the ratio of 1 : 2.2, and the higher concentration of 1 : 4.8 exceeded the optimum range, resulting in the specific surface area of 92 m 2 /g, which means a massive drop in that ratio, hence this ratio did not seem to be an efficient try.
There are several reports indicate that the higher concentration of NaOH in the fusion step may result in more stable product and higher crystallinity. [39,54]Increase of the alkali agent also has effect on the formation of specific type of zeolite from CFA at the same condition and CFA source. [55]In this study, however, the optimum range was selected upon the highest range of obtained surface area.
According to Murukutti and Jena, higher specific surface area makes the synthesized zeolite suitable for the ion exchange process, which may reduce LDPE cracking temperature compared to those with a lower surface area. [14,41]In this regard, Serrano et al. [56] highlighted the bulky nature of polyolefins (such as LDPE) which cause the occurrence of steric and diffusional hindrances for going into the zeolite pores.This report call attention of having easily accessible acid sites, through a high specific surface area or larger pore distribution on the synthesized zeolites.
The X-ray diffraction (XRD) patterns of the raw CFA and its crystalline phases such as quartz, mullite, hematite etc. discussed above.Figure 4 shows the XRD pattern for synthesized materials over the NaOH ratio of 1.2, 1.8 and 2.2 within the 2θ of 5°to 40°.At this stage the sample with the NaOH to CFA ratio of 4.8 was excluded since it exceeded the optimum range of the specific surface area.
Considering the ratio of 1.2, the XRD pattern indicates mostly amorphous features that resulted from transformation of CFA. Figure 5(a) shows the SEM micrographs of this sample where amorphous clusters could be clearly observed.There are also unreacted CFA particles which was not detected in X-ray diffraction but can be seen on SEM analysis.
In the following samples, co-crystallization of zeolite A along with the X phase was observed in those with higher NaOH ratios of 2 and 2.4.The fingerprint line of zeolite A (2θ = 7.20 and 9.93) and zeolite X (2θ = 6.10) can be seen on the samples' XRD patterns which confirms formation of zeolitic material. [57]igure 5(b) and (c) exhibit SEM micrographs of the prepared samples confirm presence of pyramidal octahedral crystals characteristic for zeolite NaÀ X and cubic crystal which related to NaÀ A type zeolite [58] which is in agreement with the XRD patterns.In the same situation, by contrast, the synthesized zeolite over the NaOH ratio of 2.4 shows a much higher degree of crystallinity where the X phase boosted up according to the XRD pattern.This sample also depicted a more specific surface area in BET analysis, and no unreacted CFA particle was found in its SEM micrograph.
Hence, the concentration of NaOH (alkali) plays a pivotal role in the synthesis of zeolite from CFA and has an imperative role in attaining the desired zeolite characteristics.
The FTIR spectra is a complementary study to the XRD method.This analysis can provide helpful information regarding zeolite minerals and their tetrahedral site. [59]FTIR spectra of the different NaOH ratio (1.2, 1.8 and 2.4), resulted in the graph represented in Figure 6.
Major difference with the original CFA can be seen from the first sight.According to Jha et al., variations of the bands between the 400 to 1200 cm À 1 , such as changes in broadness and intensity, confirm their amorphization, change in mineralogy and morphology, generation of crystalline silicate and formation of primary and double ring secondary building units (i.e. fly ash zeolites). [60]he strongest vibration band appeared around 965-970 cm À 1 for all three samples which is due to the internal tetrahedron vibrations, detected in all zeolites attributed to a TÀ O stretching mode.The second strongest band is also common for all three samples in 430-460 cm À 1 which is assigned to a TÀ O bending vibration and the intensity of this band does not relate to the degree of crystallinity. [59]Bending vibration of water around 1645 cm À 1 and hydrogen-bonded OH to oxygen ions around 3360-3390 cm À 1 prove the presence of zeolite water in all three samples.
Another band raised in the area between 556-572 cm À 1 for all three samples.This peak indicates the phase change from amorphous into high crystalline zeolite X which is sharp for ratios of 2 and 2.4, but very weak for 1.2, in tune with XRD and SEM analysis.
Another new band appeared around 750 cm À 1 for ratios of 2 and 2.4 that is assigned to the symmetric stretching vibration band of OÀ T-O groups.This is another range in which NaX zeolite are formed. [61]elocation and changes in the intensity of bands were observed when comparing the FTIR spectra of the samples and this was mainly due to impurities and an increase in crystallinity with NaOH higher ratios.Zeolite A, for example, is considered as one of the impurities when zeolite X is the target product.Zeolite A, as an impurity, causes considerable shift of the double-ring vibration (500-650 cm À 1 ) towards a lower wave number due to its double fourring units in place of the double six-ring zeolite X. [62] According to Musyoka et al., the unique hierarchical morphology of the zeolite X delivers significant thermal stability, [62] making this type of zeolite suitable for applications that need elevated temperatures, such as LDPE pyrolysis.To evaluate the produced zeolite behavior at higher temperatures, TG analysis was conducted for the zeolite XA achieved over the NaOH ratio of 2.4 (sample with the highest specific surface area) in this study.The analysis could deliver an appropriate estimation of the performance of this product as a catalyst at elevated temperatures.
According to the TGA graph in Figure 7, weight loss began at 40 °C (from the beginning of the test) and continued up to around 250 °C.This can be attributed to the loss of water on the surface as well as water adsorbed inside the pores.The thermograph shows that the amount of moisture desorption for the synthesized zeolite is approximately 22 % which is very close to that of commercial zeolite X, which has 23 % moisture desorption. [63]That may be due to the octahedral morphology of the zeolite produced using a NaOH ratio of 2.4, which is the same as that of the commercial one.Additionally, the TGA test was conducted up to 600 °C, which is much higher than the degradation temperature of LDPE, and no further weight loss was observed.

Synthesis of H-XA (zeolite base solid acid catalyst)
The synthesized zeolite with NaOH ratio of 2.4 (surface area: 431 m 2 /g) was selected for protonation process.At this step H + ions were generated on the surface and pores of the produced zeolite, converted that into the protonated form.The protonated zeolite was named H-XA, and its acidity and catalytic activities were investigated.

Acidity
Beer's Law in FTIR spectroscopy establishes a logarithmic relationship between transmittance (%) and substance concentration, providing a basis for quantitative analysis. [64]According to Gabrienko et al. (2018), integrated absorbance of hydroxyl groups in zeolites exhibits a linear relationship with the  concentration of these groups. [65]This linear correlation between absorbance and wave number cm À 1 enables for an approximate estimation of H-XA acid site were generated during protonation and quantities by comparing it to a known zeolite sample, such as H-ZSM5 (Figure 8).
H-XA FTIR hydroxyl group region shows an absorbance of a peak in the low frequency area started around 3710 cm À 1 may relate to silanol group (Si-OH) from internal structure that weakly bonded to H and perturbed OH groups and specifically occurs on the faujasite-type zeolites. [66,67]This peak, exclusively raised in the H-XA spectra, suggests that it is generated during the protonation step.This observation could potentially be one of the reasons explain why H-XA leads to a reduction in LDPE degradation temperature, whereas no such effect is observed with Na-XA 2.4 (Figure S2).
In both the H-XA and H-ZSM5 spectra, additional peaks are observed within the central-frequency range of zeolites (3600-3650 cm À 1 ).These peaks can be attributed to bridged SiÀ O-(H)À Al groups or potentially indicate the presence of strong Brønsted acid sites. [65]H-ZSM5 exhibits a clearly defined and precisely localized peak within the frequency spectrum, which is notably sharper than the one observed for H-XA.This observation strongly suggests a significantly higher concentration of Brønsted acid sites on H-ZSM5 when compared to H-XA.

Thermal behavior
The effect of H-XA on the LDPE degradation temperature was evaluated by Thermo Gravimetry Analysis (TGA) after drawing a comparison between LDPE thermal degradation, LDPE mechanical mix (1 : 1) with CFA, LDPE with CFA-based zeolite (Na-XA 2.4) and LDPE with the protonated CFA-based zeolite (H-XA) (Figure S2).
TG weight loss data converted into normalized conversion α = (m 0 -m t )/(m 0 -m f ). Figure 9(a) depicts the profile of the normalized conversion of LDPE as a function of temperature with and without catalysts.Single step mass loss of LDPE can be inferred from all diagrams; however, the form and rate of the conversion seems different when protonated zeolites added to the system.It can be seen that the LDPE thermal degradation occurred within the range of 405 to 497 °C with the minimum DTG temperature of 476 °C which is in agreement with the reported results for this polymer. [68,69]t can be seen from Figure 9(b) the LDPE DTG curve is mainly superimposable to those mixed with CFA and Zeolite (Na-XA 2.4).However, the DTG curve of LDPE mixed with solid acid catalysts (commercial H-ZSM5 and H-XA) shows a considerable shift to a lower temperature range.Degradation took place at a broader temperature range in the presence of H-XA, i. e., 300-405 °C and the minimum DTG point of 378 °C, which means around 95 °C reduction in the degradation temperature due to the generated acid sites on the synthesized zeolite.The wide degradation temperature range and shoulder on the peak can be explained by the assumption that the mix of zeolite A and X affect the degradation mechanism in a different way and a pre-cracking stage occurs involving A or X type zeolite, which is followed by the decomposition of the initial released  compounds such as oligomers by the other part of the mixture.It is also noteworthy that the shift toward the lower degradation temperature is correlated to the amount of used catalyst so that a lesser shift occurred when a smaller ratio of the H-XA was used in the process.On the other hand, only a portion of the synthesized material is real zeolite and some impurities also exist since precursor used had plenty of unwanted material.
The LDPE degradation process with the commercial H-ZSM5 began at 332 °C and ended at 390 °C and a minimum DTG of 375 °C.Use of H-ZSM led to the narrowest degradation temperature range; however, the minimum DTG point is 375 °C which is almost the same as H-XA.

Kinetic study
The DTG curve of LDPE and the mix of LDPE and catalysts were analyzed based on the achieved TGA data (Figure S3) at four different heating rates, including 5, 10, 15 and 20 °C/min.(Figure 10a, b, c).
LDPE mass conversion as a function of temperature was also plotted, where mass conversion (α) was calculated from Eq. (3) (Figure 11a, b, c).
Model-free methods were employed to calculate kinetic parameters based on the results achieved from the thermogravimetric analysis.The activation energy (Ea) was obtained using Kissinger and KAS methods.
For the Kissinger method, E a and A were calculated by plotting ln β/T p 2 versus 1000/T p where the T p (temperature (K) at maximum weight loss peak) achieved from DTG curves on Figure 10 and β is the heating rate (5, 10, 15 and 20 °C/min).
Graphs of the regression lines is shown in Figure 12 along with the corresponding regression equations and square correlation coefficient (R 2 ).The graphs indicate a near linear relationship between the natural log of the reaction rate and temperature.The R 2 values for the linear fits are close in the Kissinger plots, which means the achieved data lay within the limits of confidence.
According to the Kissinger method, the slope of plotting the regression line equals -Ea/R, and the intercept has an equal value of ln (AR/Ea) (ASTM E 2890).The obtained activation energy for the degradation reactions by the Kissinger method is shown in Table 4.
The required energy for different conversion factor (α) values also were calculated using Eq. ( 5) Kissinger-Akahira-Sunose (KAS) method for the values of conversion (α) from 0.1 to 0.8.
Values below 0.1 are not reported here as they did not show a linear behavior.
Figure 11 depicts conversion values as a function of temperature for thermal and catalytic degradation of LDPE at four different heating rates.The corresponding temperatures of the selected conversion values were extracted from the conversion diagram and the KAS diagrams of ln(βi/T αi 2 ) versus 1000/T αi were plotted (Figure S4).In the following, the squares of the correlation coefficients, as well as the slope and intercept, were  obtained from the regression lines and he apparent activation energies was calculated from the slopes (Table 4).
The Squares of the correlation coefficients, R 2 , correspond to linear fittings are close for all values from 0.991 to 0.999 (α = 0.1-0.8).
The average values of activation energy obtained from the KAS method are in tune with the range of Ea values calculated by the Kissinger method; however, it can be inferred from the results that LDPE catalytic degradation is a multi-step reaction and the activation energy with the presence of catalysts is a function of conversion level.
Moreover, it can be estimated that impurities among waste LDPE can impact the conversion rate and, consequently, the activation energy.This matter is vital from the industrial and economic point of view where a constant output from the waste LDPE degradation processes are expected.
Figure 13 shows activation energy as a function of LDPE conversion with and without the catalysts obtained through the KAS method.
It can be inferred from the graphs that the thermal degradation of LDPE requires a substantially higher level of energy compared to the catalytic process.Fluctuations in the LDPE thermal degradation are negligible, while they are considerable in catalytic decompositions, which means distinct degradation mechanisms at different conversion rates.The activation energy downward trend for LDPE 1 : 1 H-ZSM5 may be explained by the stable acidity of H-ZSM5 and the escalation of its activity under the effect of higher temperatures.The rising trend for LDPE 1 : 1 H-XA could be due to the H-XA limited acidity, which was used and lessened at the time of the reaction.

Pyrolysis GC-MS of LDPE
The catalytic activity of the using acid catalysts was tested to evaluate their potential to lead the degradation output.For this purpose, pyrolysis coupled with gas chromatography separation and mass spectrometry detection (PyÀ GC/MS) were used to investigate the nature of evolving products from LDPE with the presence of the synthesized acid catalyst.The efficiency of this method was tested by Serrano et al., and the results are comparable to the tests held in reactors and reported by other authors. [18]However, the very short residence time of the emitting volatiles in the pyrolyser and transfer line to the gas chromatography does not allow the occurrence of side reactions. [70]Thus, the effect of side reactions is neglected in this method, and the final output may differ from the typical reactors and cracking systems.
On the other hand, this method may not be efficient for waste LDPE, as the amount of sample is very small (~0.2 mg) in this technique, and the presence of impurities and unwanted  material may not be the same for different specimens, which means a high scattering in the results.Figure 14 depicted the GC-MS chromatograms of LDPE sample in the form of powder, and the 1 : 1 mechanical blend of LDPE and the catalysts.At the first sight, heavier hydrocarbons disappeared when degradation conducted in the presence of acid catalysts.It can be inferred from this observation that most of the degradation outputs were reformed when using catalysts acid sites.
It also suggested that the expected LDPE mass and heat transfer hindrance had minimum effect on catalytic degradation as most volatiles could readily access the catalysts' acid sites.Notably, the pyrolyser heating rate (10 °C/ms) did not let the inner part of LDPE particles contact acid sites directly and was only affected by the thermal degradation.Nevertheless, the condition of the system used in this study, exposes the generated volatile to the catalysts acid sites, which may not occur in typical reactors and the scale of accessibility depends on the morphology of the using catalyst. [71]urther examination of the LDPE pyrogram (Figure 15a) shows the LDPE typical triplet peaks were identified, comprising a diene, an alkene, and an alkane of a specific chain length.Smaller molecules (C 2 À C 5 ) detected at the retention time before 5 min consist of olefins, linear and branched types of paraffin and cycled compounds, but no formation of aromatic except a small amount (~0.3 %) benzene within this area.
Figure 15b shows the catalytic degradation of LDPE over the synthesized protonated zeolite (H-XA) in this work.The catalytic degradation caused the elimination of heavier hydrocarbons, more than C 9 , while the formation of gaseous hydrocarbons and light aromatics such as benzene, toluene and xylene promoted.
Therefore, it can be concluded that the average hydrogento-carbon ratio (H/C) increased when we utilized the synthesized catalyst during the pyrolysis of LDPE.This increase in the H/C ratio implies the generation of outputs with lower density.This is because a higher H/C ratio often indicates a lower proportion of carbon atoms, which have a greater atomic weight than hydrogen atoms.As such, hydrocarbons with less carbon atoms will generally have a lower molecular weight.
In this regard, Wang et al. have indicated, the coupling of the H/C molar ratio and molecular weight (M) provides a useful means for estimating various properties such as density, net heat of combustion, viscosity, and serves as an effective tool for the design, manufacturing, and evaluation of fuels.However, it's crucial to recognize that this approach represents a simplification.The actual molecular weight can also be influenced by the specific arrangement of atoms within the molecule, known as the compound's specific structural isomerism.Different isomers of a hydrocarbon may share the same number of carbon and hydrogen atoms but exhibit varying molecular weights due to distinctions in their structural arrangements. [72]atalytic degradation of LDPE over H-ZSM5 is shown in Figure 15(c).Cracking of large LDPE molecules into C 3 to C 7 range of hydrocarbons as well as forming aromatics which, due to the hydrogen transfer reactions [73] were observed in the results.
Comparing the outputs and their retention time over H-XA and H-ZSM5 indicate comparable catalytic activity of H-XA and H-ZSM5.These results are in agreement with the DTG curves obtained in the thermal analysis section.

Comparison of H-XA with a commercialized product and publicly available literature
Table 5 shows an evaluation of seven different catalysts which have been used for LDPE decomposition processes.The table compares products that synthesised in this work with its counterpart in other studies of this kind and a commercial H-ZSM5 which has been used in several works.
Comparing the product properties and their catalytic activity put H-XA among the most active catalysts in this group.The similarity of results between the commercial H-ZSM5 and H-XA, for example, is noticeable and suggests the great potential of the proposed method of this study as an uncomplicated and economical approach for upscaling.
Table 6 depicts cost estimation for production of 1 kg H-XA at laboratory scale and is based on expenses in Victoria, Australia.The cost estimation would create a better view of the economic aspects of this work and enable a better comparison with other available products in the market.The estimation was conducted based on the cost of materials, electrical consumption and delivery of materials offered in Victoria, Australia.
It is noticeable that the factors such as land, enterprise and labor have not been considered in the cost assessment, and it is only based on what can take place on a laboratory scale (Table S1 & S2).On the other hand, upscaling and mass production might be more cost-effective, especially through optimization based on production factors.

Conclusions
In this study, coal fly ash (CFA) was converted to a blend of A and X zeolite by way of alkali fusion, followed by sonication and pressure-free hydrothermal treatment.Synthesised material then received a protonation treatment and turned into a solid acid catalyst called H-XA.
Additionally, H-XA and a commercial H-ZSM5 were mechanically mixed (ratio 1 : 1) with pure LDPE as one of the primary sources of single-use plastics and tested for their catalytic activity.The presence of H-XA caused a significant reduction in the required endothermic energy by the LDPE degradation reaction, comparable with commercial H-ZSM5.
To further investigate the catalyst activity, pyrolysis coupled with gas chromatography and mass spectrometry was employed to quantitively analyze the nature of output emitted by LDPE thermal and catalytic degradation.Results indicated that the presence of H-XA and H-ZSM5 caused the elimination of heavier hydrocarbons and significant shape selectivity toward gas products.
Evaluating the synthesised catalyst's performance on other plastics' degradation seems beneficial for developing the circular economy of plastics.Therefore, study on catalyst-to-plastic ratio, catalyst recovery and reactor layout are essential for future improvements.

Materials
Low density polyethylene with the maximum particle size of 300 μm from Goodfellows (Cambridgeshire, England) was used in this study.As per the datasheet from Goodfellow, the LDPE sample has a density of 0.920 g/cm 3 .This relatively low density suggests a high degree of branching in the polymer chains, leading to a less tightly packed molecular structure.The inherent flexibility resulting from this branching makes this LDPE sample well-suited for applications requiring adaptability to various shapes, particularly in uses like packaging films. [80]lass F coal fly ash (CFA) received from Boral Australia (construction material company) with silica alumina molar ratio of ~4.6 according to the manufacturer website.
The sodium hydroxide from Sigma Aldrich (reagent grade, 97 %) was utilized in the alkali fusion step and Ammonium chloride (NH 4 Cl) from Sigma Aldrich was used to ion exchange and protonation step of this work.
Commercial Protonated Zeolite Socony Mobil-5 (HZSM-5) purchased from ACS Material, (CA, United States) with SiO 2 / Al 2 O 3 molar ratio of ~26 and BET surface area of 352 m 2 /g for the purpose of comparison and evaluation of the catalyst's activity in this study.

Zeolite synthesis
][78][79][80][81] Alkali fusion followed by hydrothermal treatment is an energy-efficient method with minimum operational complexity [81] promising to upscale; hence the method was selected to synthesize zeolite with some modifications to make it more efficient and less complex.
The synthesis of zeolite through selected methods involved numerous variables such as the alkali agent to CFA weight ratio, temperature and time in different steps.After several trials and errors, the optimal synthesis parameters were determined and the impact of the NaOH to CFA ratio on the quality and nature of the generated zeolite, as well as their catalytic activity in the pyrolysis of LDPE, was studied.
For CFA pre-treatment, two hours of calcination at 800 °C (air atmosphere) was performed then the sample was sieved to achieve a particle size below 38 μm, which was confirmed through particle size analysis by Malvern Mastersizer 3000.Primary calcination is an optional step that can be effective in Total [a] 141.24 USD/kg [a] The total cost before protonation process up to the production of Na-XA zeolite was USD 80. removing unburned carbon particles from CFA. [45] Acid treatment did not perform in this work as a pre-treatment step but is recommended if impurities are considerable. [79]ext, the zeolization started following the fusion of NaOH/ CFA over different ratios of 0, 1.2, 1.8, 2.4 and 4.8 for 1 h at 800 °C in air atmosphere.The fused material then milled to powder and dissolved in distilled water with a liquid to solid ratio of 10 : 1 followed by 18 h of stirring on a hotplate at 40 °C.The resulting slurry then sonicated for 1 h using a bath sonication (Power Sonic 405) at its highest level then placed in an oven for a 6 h incubation at 90 °C with the purpose of crystallization.Finally, the solid part filtered, washed to the neutral PH, and dried over night at 100 °C (Figure 16).

Characterization of the synthesised zeolite
Samples characterization started by measurement of specific surface area and pore size by theories of Brunauer-Emmett-Teller (BET) and Barrett, Joyner, Halenda (BJH) with its specific instrument (Quantachrome automated gas sorption analyzer).
Zeiss SUPRA 55-VP FEGSEM instrument used to realize the morphology and microstructure of raw CFA and the synthesised samples and take SEM images.Fourier-transform infrared spectroscopy (FTIR) analysis was carried out with an IR (Bruker ALPHA) spectrometer for a better understanding of the nanostructure and tracking the scale of reactions and changes from CFA to the synthesised Zeolite.XRD patterns also were identified by X-ray diffraction in an X'Pert 3 Powder (PANalytical) in the range 5°-60°which helped to measure the crystallinity of the synthesized samples.
Furthermore, the synthesised zeolites thermal behavior and stability was required as the zeolite will be used as a catalyst in the pyrolysis elevated temperature or future heat treatment for catalyst regeneration.Thus, Thermogravimetric analysis (TGA) was conducted (NETZSCH -TG 209 F1 Libra) with the heating rate of 10 °C/min in a N2 atmosphere to compare the synthesised zeolites with its commercial counterpart.

Protonation of the synthesised zeolite
As mentioned before acidic sites enhance carbocationic cracking and hydrogen transfer and promote catalytic pyrolysis cracking reactions. [26,82]Therefore, acidification of the synthesised zeolite carried through ion exchange of Na-zeolite with an aqueous solution of an ammonium salt (NH 4 Cl).Aiming to exchange Na + with NH 4 + ions the process started by loading of the synthesised catalyst into the 2 M solution of NH 4 Cl with the solid to liquid mass ratio of 1 : 4. The slurry stirred for 72 h under a knockback condenser at 100 °C.According to Chu, a higher temperature (100 °C) and fresh ammonium sources are required to achieve a more efficient ion exchange. [83]Hence, a fresh NH4Cl solution with the same molarity and mass ratio was replaced with the old one every 24 h.At the end, the solid part filtered, washed, and dried over night at 100 °C then calcinated under nitrogen flow for 4 h at 400 °C which caused removal of ammonia (NH3) and generation of HCFA zeolite (Figure 17). [84]idity measurements Acidity of zeolite-based catalysts, particularly Brønsted acid sites arising from the zeolite SiÀ O(H)À Al bridging hydroxyl groups has an important role in their activity.[20,85,86] Characterization of zeolite acidity is challenging due to the complexity in zeolites structure and different methods have been studied with pros and cons.One of the promising method for characterization of zeolite hydroxyl groups is transmission FTIR spectroscopy that enables direct detection of ν OÀ H stretching vibrations.[65] Three ranges of frequency were identified in hydroxyl stretching region upon the FTIR analysis of H-forms of zeolites in the dehydrated state including high, central, and low.Highfrequency originated from silanol groups (SiÀ OH) on the external surface of the crystal can give rise to broad bands within 3200-3650 cm À 1 when strongly bonded to H. Centralfrequency usually comes from wide pores or large cavities on the zeolite structure and find source in acidic bridging hydroxy groups, generating bands falling within the 3600-3650 cm À 1 range.low-frequency range, however, originated from perturbed OH groups and usually resides within the sodalite cages of faujasite-type zeolites.It is worth mentioning that silanols  can also be found in internal positions, where they undergo slight electrostatic influences.These factors result in a shift towards lower frequencies, typically falling within the range of 3700-3720 cm À 1 .[66,67] The designation of the ν OÀ H bands in zeolites to specific hydroxyl groups and acid sites have been monitored in several studies and accepted, however, quantification of the hydroxyl groups by FTIR method is challenging. The quantiication of hydroxyl groups or acid sites requires an integrated coefficient of the molar absorption, which is at the center of several studies.For example, Gabrienko et al. reported coefficients (ɛ) to be 3.06 � 0.04 and 1.50 � 0.06 cm μmol À 1 for the IR bands at 3605-3615 and 3740-3747 cm À 1 , respectively.[65] For a quantitative evaluation of hydroxyl groups in the protonated sample, an FTIR (Fourier Transform Infrared) test was conducted under identical conditions for the synthesised zeolite (Na-XA 2.4) after protonation (H-XA) and commercial H-ZSM5.The resulting data was analyzed, and a comprehensive graph was generated for the hydroxyl group region to illustrate the relationship between absorbance and wavelength cm À 1 .

Thermal Analysis
Thermogravimetric analysis was carried out using TG 209 F1 Libra from Netzsch to investigate the behavior of LDPE during thermal degradation and the effect of catalyst on the degradation temperature and activation energy.For this purpose, a total mass of 15.0 � 0.5 mg of a LDPE or mixture of the LDPE: catalyst with the ratio of 1 : 1 (selected based on optimum performance in lowering the degradation temperature) placed in an alumina sample pan.The crucible then set into the testing machine and heated up to 650 °C with the heating rate of 10 °C/min and nitrogen flow of 60 mL/min.

Kinetic modelling
The kinetic of the LDPE pyrolysis and catalytic decomposition is also investigated using thermogravimetric results.LDPE and LDPE : catalysts mix (1 : 1) were heated from 40 to 650 °C with four different heating rates of 5, 10, 15 and 20 °C/min.
There are two classifications for kinetic analysis techniques, either iso-conversional upon multiple heating rates (model-free) or model-fitting methods (kinetic reaction model identification), mainly involving a single heating rate.The advantage of the model-free technique is flexibility against the change of mechanism during the reaction time and is able to reduce mass transfer limitations as it uses multiple heating rates.By contrast, the mass and energy transfer cause variation in activation energy at different heating rates, which the model fitting kinetic method does not consider. [87]he pyrolysis and catalytic degradation reaction of LDPE is expressed by the (equation 1) Where the Rate constant (k) can be calculated by Arrhenius equation ( 2) A: Frequency or pre-exponential factor, Ea: Activation Energy, R: The gas consent, T: Temperature (Kelvin) And F(α) is LDPE conversion function which is upon reaction rate mechanism and control the overall process; α is the fractional mass conversion (equation 3) m 0 : sample mass (LDPE or mix of LDPE : catalyst), mf: The final mass, mt: The mass of sample at time t Several kinetics modellings have been reported to calculate Ea (activation energy) and A (pre-exponential factor).However, TG data and DTG graphs were used in this study to obtain the kinetic parameters.The model-free (Iso-conversional) kinetic analysis techniques, i. e. Kissinger, KAS and OFW, were selected to obtain and compare the kinetic parameters.
Kissinger's method is one of the model-free, non-isothermal methods assumes that activation energy has a constant value for a specific conversion (equation 4) À � [88]   and intercept is equal to ln(AR/E a ).The Kissinger-Akahira-Sunose (KAS) model is another isoconversional method mostly use for kinetics of pyrolysis reactions. [89]This method works upon the following equation ( 5) and the mathematical form only enable the calculation of activation energy (E α ) as the kinetic parameter and determination of the pre-exponential factor (A α ) is not reliable. [90]Kissinger method, however, can be used for the estimation of pre-exponential factor at the extent of α when the value of Eα is calculated from iso-conversional techniques. [91] In this method activation energy can be achieved by plotting lnð b i T 2 ai ) as a function of 1000 T ai for a specific value of α (conversion), where the slop of the plot equals À E a R .

Py-GC/MS Analyses
Apart from thermogravimetric analysis and parameters such as apparent activation energy that acquired by the kinetic modelling, Pyrolysis-gas chromatography-mass spectrometry (Py-GCMS) technique was put in practice to investigate the volatile output that released during the pyrolysis process.
In view of this, CDS-6150 was used in this study as the pyrolyser hyphenated with an Agilent 6890 N Gas Chromatograph and Agilent 5973 N Mass Selective Detector.Around 0.2 mg of the utilized LDPE powder placed inside a quartz tube (Ø25 mm) for the thermal cracking.In thermocatalytic approach, LDPE first mechanically mixed with the using catalyst in the ratio of 1 : 1, around 0.2 mg of the mixture then placed inside the quartz tube for the test.
The filled quartz tube was placed inside the pyrolyser chamber and heated up to 700 °C with a rate of 10 °C/ms and a holding time of 30 sec.The temperature of the transfer line, which led the generated volatiles to the gas chromatograph (GC) set at 300 °C, and GC was set to operate at a continuous helium flux (1.0 ml/min) and a 1 : 100 split.The GC oven program started at 32 °C for 2 min then heated up to 315 °C with a rate of 6 °C/min and a final hold time of 15 minutes.
Mass detector adjusted to the ionization energy of 70 eV, scan mode between 25 and 550 amu (atomic mass unit) at 2.78 scans per second.The Agilent ChemStation software was then put in use to match the evolved species extraction time with those available as the standard.
Each trial was performed at least twice to confirm the reproducibility of the results; through the reported mechanical mixed approach.

Figure 2 .
Figure 2. The SEM images of the CFA.

Figure 4 .
X-ray diffraction patterns of the synthesized material NaOH to CFA ratio of 1.2, 1.8 and 2.4.XRD patterns confirmed no trace of CFA minerals in the synthesized zeolite patterns.However, the SEM micrograph of the NaOH ratio of 1.8 Figure 5(b) shows unreacted CFA particles which have cracks on the surface and are covered by the mineral deposits.

Figure 6 .
Figure 6.FTIR spectra of the synthesized material NaOH to CFA ratio of 1.2, 1.8 and 2.4.

Figure 7 .
Figure 7. TGA curves of CFA-based zeolite synthesized over NaOH ratio of 2.4.

Figure 13 .
Figure 13.Comparison of activation energy, obtained by KAS method for different values of conversion.

Figure 14 .
Figure 14.Py-GC/MS plot for the thermal and catalytic degradation of LDPE.

Figure 16 .
Figure 16.Schematic process flow diagram showing synthesis process of zeolite from coal fly ash.

Figure 17 .
Figure 17.Schematic process flow diagram showing protonation of the synthesised zeolite and analyses.

Table 1 .
CFA particle size analysis before and after pre-treatment.

Table 2 .
BET analysis of coal fly ash before and after the initial treatments.

Table 3 .
Fly ash and synthesized catalysts BET surface area and pore size.

Table 4 .
Results of Eα obtained by KAS and Kissinger methods, R2 corresponding to linear fittings in KAS diagrams.

Table 6 .
Estimation of 1 kg H-XA production cost in laboratory.

Table 7 .
Comparison of the zeolites' cost.
Heating Rate, T p : maximum temperature of the DTG curve (K), E a : Activation Energy Kissinger's method introduces a plot in which the slope of β: